hmmmm . . .
This is a great question. "Seasoning" is a word that gets thrown around a lot in world of cooking. Whatever it is, it seems to be the thing that distinguishes the good food from the not-so-great.
I recently listened to a podcast of Thomas Keller on splendid table, and he defined "seasoning" as anything used to enhance the flavor of an ingredient or dish, without being distinguishable in and of itself.
He then narrowed seasoning down to two things: salt and acid (vinegar, lemon juice, etc.) As opposed to say . . . black pepper, whose distinctive flavor asserts it self in addition to flavors that are already present.
To elaborate, say there you have a flavorful soup. You slurp a spoonful and the taste is there, but it's not quite amazing. You add a healthy pinch of salt and a few drops of lemon juice and NOW your soup tastes amazing. You pick up on all of the flavors, without detecting the distinct flavor of lemon or salt.
I find this to be where most cooks, professional and amateur, go wrong. They have the ingredients and make the effort, and even build good flavors, but they're simply not enhanced with enough seasoning.